Fix /usr/bin/microb script to work with new Fremantle MicroB launching code

The new Fremantle MicroB launching code breaks the assumption in the
microb script that the existence of the MicroB browser process implies
that MicroB is handling com.nokia.osso_browser.  Revise the script to
check directly whether Browser Switchboard is handling that name, by
comparing its owner to that of org.maemo.garage.browser-switchboard
(using the org.freedesktop.DBus.GetNameOwner method).